A spin-transfer torque oscillator is an oscillator which generates specific frequency signal resulted from spin precession inside magnetic-nonmagnetic multilayer structure by applying current to it. The spin-transfer torque oscillator has several advantages compared to prior art oscillators. These advantages are the nanometer scale structure fabrication using CMOS manufacturing process and the generation of wide range signals from a few MHz to GHz by varying input current. However, disadvantages of the spin-transfer torque oscillator are very low output levels in the range of nV and high phase noise characteristics.
The prior art inventions describe injection locking and mutual locking techniques for phase locking of spin oscillator as disclosed in the non-patent reference 1, 2, and 3. In this regard, the non-patent reference 1 reported that two nano-contact spin torque devices in the mutual phase locking are synchronized with a sudden narrowing of signal line-width with an increase in output power due to the coherence of the individual oscillators. However the output power of the oscillator is still in the range of a few pW, which is not suitable for telecommunication circuits. Injection locking uses locking or pulling phenomenon of periodic signals inside an oscillator as disclosed in the non-patent reference 2 and 3 experimentally and theoretically. The phase noise is reduced by locking the oscillating frequency into injection frequency. This is called as injection locking which makes the signal attenuation when input signal and oscillator signal having phase differences are combined, and at that time the phase has to vary to synchronize the phase of oscillator to the phase of injection signal for the maintaining of oscillation. However, the locking range in the silicon CMOS manufacturing process is very narrow and the phase noise reduction effect of the injection locking lacks of reliability due to the lack of suitable models and manufacturing process change. Moreover, the nanoscale integration advantage may disappear because additional oscillator is needed for injection locking.